1. Field of the Invention
The present invention relates to a method of producing a surface acoustic wave device which is packaged by a flip-chip bonding method, and in particular, the present invention relates to a method of producing a surface acoustic wave device in which an electrode is provided on a piezoelectric substrate, the electrode having a portion thereof in which plural electrode layers are laminated, and an insulation layer is formed so as to cover the electrode, and also relates to the surface acoustic wave device.
2. Description of the Related Art
Different types of surface acoustic wave devices, which are mounted onto packages by a flip-chip bonding method, have been proposed. In some of the surface acoustic wave devices, a portion of the electrode has a structure in which plural electrode layers are laminated to each other. For example, in some cases, in a wiring electrode in contact with an interdigital electrode, plural electrode layers are laminated to each other in order to reduce the resistance. Moreover, in some cases, an electrode pad in contact with the wiring electrode has a structure in which plural electrode layers are laminated to each other, so that an impact against a substrate occurring during bump-bonding can be relaxed. Moreover, in some cases, the bus bar of an interdigital electrode has a structure in which plural electrode layers are laminated to each other. Thereby, the confinement effect is enhanced, the insertion loss is reduced, and the pass bandwidth is increased.
Moreover, in some cases, in these types of surface acoustic wave devices, an insulation film made of SiO2, SiN, or the like is formed so as to cover the electrodes except for the bump-bonding portion for the protection of the electrodes and the frequency-adjustment.
Japanese Patent No. 3435639 (Patent Document 1) describes an example of the above-described methods of producing surface acoustic wave devices. The method of producing a surface acoustic wave device of Patent Document 1 will be described with reference to FIGS. 6A, 6B, 6C, 7A, 7B, and 7C.
First, a first electrode layer 102 is formed on a piezoelectric substrate 101, as shown in FIG. 6A. Subsequently, a second electrode film 103 is formed on the first electrode layer 102, as shown in FIG. 6B. The second electrode layer 103 is formed on a portion of the upper surface of the first electrode layer 102, not on the entire upper surface thereof. That is, the second electrode layer 103 is provided only in the area where the above-described electrode pad, wiring electrode, and bus bar are to be formed. Thus, a wiring electrode 104, an electrode pad 105, and a bus bar 106 are formed in which the first and second electrode layers are laminated to each other. An electrode layer 107 is a part constituting an electrode finger of the interdigital electrode. Thus, the electrode finger constituting part 107 is composed of the first electrode layer 102 only.
Subsequently, a bump 108 is bonded onto the electrode pad 105 as shown in FIG. 6C.
Then, an insulation film 109 made of SiO2 is wholly formed thereon, as shown in FIG. 7A. Then, the insulation film 109 is etched so that the bump 108 is exposed as shown in FIG. 7B. Moreover, the insulation film 109 is etched so as to have a reduced thickness as shown in FIG. 7C. Thus, the frequency characteristic is adjusted.
Moreover, Japanese Unexamined Patent Application Publication No. 10-163789 (Patent Document 2) discloses a method of producing a surface acoustic wave device which is different from one described in Patent Document 1. The surface acoustic wave device disclosed in Patent Document 2 will be described with reference to FIGS. 8A to 8C and 9A to 9C.
First, a first electrode layer 202 is formed on a piezoelectric substrate 201, as shown in FIG. 8A. Subsequently, a second electrode layer 203 is formed on the first electrode layer 202, as shown in FIG. 8B.
The second electrode layer 203 is provided only in the area where an electrode pad, a wiring electrode, and a bus bar are to be formed.
Thus, a wiring electrode 204, an electrode pad 205, and a bus bar 206 are formed. An electrode layer 207 corresponds to a portion constituting an electrode finger of an interdigital electrode.
Then, as shown in FIG. 8C, an insulation film 208 made of SiO2 is laminated on the entire surface. Subsequently, as shown in FIG. 9A, the portion of the insulation film 208 existing on the electrode pad 205 is removed by etching. Then, as shown in FIG. 9B, the insulation film 208 is etched so as to have a reduced thickness. Thus, the frequency characteristic is adjusted. Finally, a bump 209 is bonded onto the electrode pad 205.
Referring to the production method described in Patent Document 1, immediately after the electrode pad 105 is formed, the bump 108 is bonded to the upper surface of the portion of the second electrode layer 103 constituting the electrode pad 105. Accordingly, the bonding strength between the electrode pad 105 and the bump 108 can be sufficiently enhanced. However, after the bump 108 is formed, the insulation film 109 is formed so that the bump 108 is covered. As shown in FIG. 7C, the insulation film 109 existing on the bump 108 is removed. However, problems occur in that, in flip-chip bonding, the bonding strength between the bump 108 and an electrode of the package may be reduced, due to the residues of the insulation film on the bump 108.
Moreover, referring to the production method described in Patent Document 2, the insulation film 208 is formed immediately after the electrode pad 205 is formed. The insulation film 208 existing on the electrode pad is removed. Thereafter, the bump 209 is formed on the electrode pad 205. Accordingly, possibly, the bonding strength between the electrode pad 205 and the bump 209 is reduced, due to the residues of the insulation film.